Melter assembly and method for charging a crystal forming apparatus with molten source material

ABSTRACT

A melter assembly supplies a charge of molten source material to a crystal forming apparatus for use in forming crystalline bodies. The melter assembly comprises a housing and a crucible located in the housing. A heater is disposed relative to the crucible for melting solid source material received in the crucible. The crucible has a nozzle to control the flow of molten source material such that a directed flow of molten source material can be supplied to the crystal forming apparatus at a selected flow rate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a non-provisional of U.S. Provisional PatentApplication Ser. No. 60/581,308 filed Jun. 18, 2004 which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to a melter assembly for melting solidsilicon, and more particularly to a melter assembly and method forcharging a crystal forming apparatus with molten source material.

Single crystal material having a monocrystalline structure, which is thestarting material for fabricating many electronic components such assemiconductor devices and solar cells, is commonly prepared using theCzochralski (“Cz”) method. Briefly described, the Czochralski methodinvolves melting polycrystalline source material such as granular orchunk polycrystalline silicon (“polysilicon”) in a quartz cruciblelocated in a specifically designed furnace to form a silicon melt. Aninert gas such as argon is typically circulated through the furnace. Arelatively small seed crystal is mounted above the crucible on a pullingshaft which can raise or lower the seed crystal. The crucible is rotatedand the seed crystal is lowered into contact with the molten silicon inthe crucible. When the seed begins to melt, it is slowly withdrawn fromthe molten silicon and starts to grow, drawing silicon from the melthaving a monocrystalline structure.

Large grain polycrystalline semiconductor structures suitable for use asthe starting material for the production of solar cells or otherelectrical components may be produced by various other processes knownin the art. As with the Czochralski method, such alternative processesinclude various apparatus that utilize molten source material (e.g.silicon) to produce a solid crystalline body (e.g. ingot, ribbon, etc.)having desired electrical conduction properties. Such processes mayinclude block casting which entails filling a cold crucible with moltensilicon and allowing the molten silicon to solidify and form apolycrystalline body. Another process, commonly known as, theEdge-defined Film Growth (EFG) method involves growing hollowcrystalline bodies in diverse shapes of controlled dimensions by usingcapillary die members which employ capillary action to assist thetransfer of molten source material from a crucible to a seed crystalconnected to a pulling apparatus. Also, various ribbon growth methodsexist that involve the growth of a generally flat crystalline ribbonstructure that is pulled from the melt of source material.

The various existing methods for forming semiconductor material forsemiconductor devices and solar cells typically include the step ofmelting granular polysilicon directly in a crucible or adding a chargeof molten silicon to a crucible. One drawback of melting granularpolysilicon directly in a crucible is that the polysilicon is preferablyhighly pure, dehydrogenated silicon to reduce splatter from the releaseof hydrogen during melting. Splatter in the crucible causes silicon todeposit on the various components of the hot zone of the crystal formingapparatus and may result in impurities in the pulled crystal or damageto the graphite and silicon-carbide coated graphite components in thehot zone. Dehydrogenated chemical vapor deposition (CVD) granularpolysilicon is expensive in comparison to more readily available CVDpolysilicon that is not dehydrogenated, and its use adds to theproduction costs of silicon wafers or other electrical componentsproduced by the various methods.

Additional operational and mechanical problems result from the meltingof solid polysilicon in the main crucible of the crystal formingapparatus. For example, a large amount of power is required to melt thepolysilicon due to its high thermal conductivity and high emissivityrelative to liquid silicon. Also, melting solid polysilicon in the maincrucible is time consuming typically requiring 15-18 hours to melt asingle 250 kg (551 lbs) charge of polysilicon. Further, the thermalstresses (both chemical and mechanical) induced in the crucible byexposure to the high melting temperatures required to melt the solidpolysilicon cause particles of the crucible walls to be loosened andsuspended in the melt resulting in lower crystal quality and prematurefailure of the crucible. Also, the crucible is subjected to mechanicalstresses from the loading of solid polysilicon particles that frequentlyscratch or gouge the crucible wall resulting in damage to the crucibleand removal of particles from the crucible walls that may contaminatethe silicon melt and the bodies formed therefrom.

Various prior art methods have attempted to eliminate the requirement ofmelting polysilicon in a crucible. These prior art methods includeproviding an auxiliary crucible for melting the polysilicon locatedabove the main crucible so that gravity feed with or without use ofdifferential pressure allows molten silicon to flow into the maincrucible during crystal growth. These existing prior art methods do notefficiently and quickly melt the solid polysilicon and do not transferthe melted silicon to the main crucible in a manner that reduces meltsplatter on the hot zone parts of the crystal forming apparatus. Also,the existing methods do not provide a quick and economical way ofheating the solid polysilicon to reduce melting time and increase thethroughput of the crystal forming apparatus. Therefore, a need existsfor a method of supplying molten silicon to a crystal forming apparatusthat quickly and efficiently melts the solid polysilicon and transfersthe molten silicon to the main crucible of the apparatus in a way thatreduces splatter and maintains the quality of the resulting siliconcrystal.

SUMMARY OF THE INVENTION

Among the several objects of this invention may be noted the provisionof a silicon melter assembly and method of operation that facilitatesthe charging of a crystal forming apparatus with molten silicon sourcematerial; the provision of such a melter assembly and method thatincreases crystal yield and throughput; the provision of such a melterassembly and method that increases quality; the provision of such amelter assembly and method that allows for rapid heating of solidsilicon; the provision of such a melter assembly and method that allowsfor the controlled heating of the silicon; the provision of such amelter assembly and method that reduces melt splatter caused by addingmelt to the crystal forming apparatus; the provision of such a melterassembly and method that contains and isolates dust and melt splattercaused by adding polysilicon to a reservoir of molten silicon; and theprovision of such a melter assembly and method that can rechargemultiple crystal forming apparatus with a single melter.

In general, the present invention is directed to a melter assembly forsupplying a charge of molten source material to a crystal formingapparatus. The melter assembly comprises a housing and a cruciblelocated in the housing. A heater is disposed relative to the cruciblefor melting solid source material received in the crucible. The cruciblehas a nozzle adapted to control the flow of molten source material suchthat a directed flow of molten source material is supplied to theapparatus at a selected flow rate.

In yet another aspect, the present invention is directed to a melterassembly for supplying a charge of molten source material to a crystalforming apparatus for use in forming crystalline bodies. The melterassembly comprising a housing and a crucible located in the housing. Aheater is disposed relative to the crucible for melting solid sourcematerial received in the crucible. A susceptor supports the crucible andpromotes heating of the solid source material in the crucible. Thesusceptor has an upper portion and a lower portion that are spaced apartto allow independent control of the heater.

Other objects and features of the present invention will be in partapparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical section of a melter assembly of thepresent invention;

FIG. 2 is a schematic fragmentary section of a prior art crystal formingapparatus in the form of a crystal puller;

FIG. 3 is a schematic fragmentary section of the melter assemblyinstalled on a lower housing of the crystal puller;

FIG. 4 is a schematic vertical section showing the melting crucible, asusceptor, and induction coils separate from the melter assembly;

FIG. 5 is a elevation of the susceptor, melting crucible, and inductioncoils separate from the melter assembly;

FIG. 6 is a cross-section taken along the plane including line 6—6 ofFIG. 5;

FIG. 7 is a cross-section taken along the plane including line 7—7 ofFIG. 5;

FIG. 8 is an exploded vertical section of the susceptor;

FIG. 9 enlarged fragmentary section of the melting crucible showing anozzle;

FIG. 10 is a schematic vertical section similar to FIG. 4 but showingthe melting crucible being filled with solid polysilicon from a feeder;

FIG. 11 is a view similar to FIG. 10 but showing initial melting ofsolid polysilicon in the melting crucible;

FIG. 11A is a view similar to FIG. 11 but showing further melting of thesolid polysilicon in the melting crucible;

FIG. 12 is a view similar to FIG. 11 but showing still further meltingof the solid polysilicon in the melting crucible;

FIG. 13 is a view similar to FIG. 12 but showing the addition of solidpolysilicon feed into the melting crucible;

FIG. 13A is a view similar to FIG. 13 but showing further melting ofsolid polysilicon in the melting crucible;

FIG. 14 is a view similar to FIG. 12 but showing further melting of thesolid polysilicon in the melting crucible;

FIG. 15 is a view similar to FIG. 14 but showing liquid silicon flowingthrough the outlet of the melting crucible and the addition of solidpolysilicon feed into the crucible;

FIG. 16 is a view similar to FIG. 15 but showing the reduction in meltheight in the melting crucible;

FIG. 17 is a view similar to FIG. 15 but showing an alternate embodimentof the melting crucible;

FIG. 18 is a schematic section of a melter assembly of anotherembodiment of the present invention;

FIG. 19 is a enlarged, fragmentary schematic section of a melterassembly of another embodiment of the present invention;

FIG. 20 is a schematic of a melter assembly of another embodiment of thepresent invention;

FIG. 21 is an exploded section of a the melting crucible of theembodiment of FIG. 20;

FIG. 21A is a schematic section of one version of a melt flow guide;

FIG. 21B is a schematic section of a second version of the melt flowguide;

FIG. 21C is a schematic section of a third version of the melt flowguide;

FIG. 22 is a schematic section of another embodiment of a melterassembly shown installed on a crystal forming apparatus;

FIG. 23 is a schematic section of another embodiment of a melterassembly showing liquid silicon being transferred between the meltingcrucible and a main crucible of a crystal forming apparatus;

FIG. 24 is a schematic fragmentary section of a prior art crystalforming apparatus in the form of an EFG crystal puller;

FIG. 25 is a schematic fragmentary section of the melter assemblyinstalled on a lower housing of the crystal forming apparatus of FIG.24;

FIG. 26 is a partial perspective of a prior art ribbon growth crystalforming apparatus;

FIG. 27 is a schematic fragmentary section of the melter assemblyinstalled on a lower housing of the crystal forming apparatus of FIG.26;

FIG. 28 is a schematic fragmentary section of the melter assemblyinstalled above a prior art crystal forming apparatus in the form of acasting crucible; and

FIG. 29 is a schematic of a melter assembly positioned for servicing afirst crystal puller in a series of crystal pullers.

Corresponding parts are designated by corresponding reference numbersthroughout the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2, a melter-assembly of the present invention,designated in its entirety by the reference numeral 1, may be used tosupply molten source material to various crystal forming apparatus thatare generally known in the art. One such crystal forming apparatus is aconventional crystal puller, generally designated CP, of the type usedto grow monocrystalline silicon ingots (e.g., ingot I of FIG. 2)according to the Czochralski method. The crystal puller CP is shown inFIG. 2 as being configured for producing silicon ingots I. The moltensource material supplied by the melter assembly of the present inventionmay be silicon or any other source material (e.g., alumina, bariumtitanate, lithium niobate, yttrium aluminum garnet, germanium, gallium,gallium arsenide, etc.) that is used in the crystal forming apparatus toproduce solid crystalline bodies.

The crystal puller CP includes a water-cooled housing including a lowerhousing LH enclosing a lower crystal growth chamber GC and an upperhousing UH enclosing an upper pull-chamber PC partially shown in FIG. 2.The upper housing UH and lower housing LH are removably connected by aflanged connection FC and a locking device LD. The upper housing UH andlower housing LH are isolated by a gate isolation valve GV. A pump (notshown) or other suitable means may be provided for drawing a vacuum intothe interior of the housing. As shown in FIG. 3, the melter assembly 1may be installed on the lower housing LH of the crystal puller CP inplace of the upper housing UH and receive solid, granular polysiliconfrom a gravity feeder GF. As will be described more fully hereinafter,the melter assembly 1 melts the solid polysilicon and transfers moltensilicon by gravity feed to the crystal puller CP or other crystalforming apparatus.

Referring again to FIG. 2, the crystal puller CP is a conventionalcrystal puller for producing monocrystalline silicon ingots I accordingto the Czochralski method. In the illustrated embodiment, the crystalpuller CP is a Model No. FX150 crystal grower furnace manufactured byKayex Corporation of Rochester, N.Y., but it will be understood thatother crystal pullers may be used without departing from the scope ofthis invention. The lower housing LH enclosing the crystal growthchamber GC has a generally dome shaped upper wall UW and houses a quartzcrystal growth crucible CC seated in a graphite susceptor S. Thecrucible CC contains molten source material M from which themonocrystalline silicon ingot I is grown. The susceptor CS is mounted ona turntable T for rotation of the susceptor S and crucible CC about acentral longitudinal axis of the crystal puller CP. The crucible CC isalso capable of being raised within the growth chamber GC to maintainthe surface of the molten source material M at a generally constantlevel as the ingot I is grown and source material is removed from themelt.

An electrical resistance heater HC surrounds the crucible CC for heatingthe crucible to maintain the source material M in a molten state. Theheater HC is controlled by an external control system (not shown) sothat the temperature of the molten source material M is preciselycontrolled throughout the pulling process. A heat shield assembly HS ismounted in the growth chamber GC above the molten source material M andhas a central opening sized and shaped to surround the ingot I as theingot is pulled up from the source material. The area in the lowerhousing LH surrounding the crystal growth crucible is generally referredto as the “hot zone” of the puller CP. The hot zone parts include thesusceptor S, heater HC, heat shield assembly HS, and heat reflectors orinsulation I that control the heat transfer around the crucible CC andthe cooling rate of the growing crystal.

A pull shaft PS extends down from a pulling mechanism PM capable ofraising, lowering and rotating the pull shaft. The crystal puller CP mayhave a pull wire (not shown) rather than a shaft PS, depending upon thetype of puller. The pull shaft PS extends downward through the pullchamber PC and terminates in a seed crystal chuck SC which holds a seedcrystal C used to grow the monocrystalline ingot I. The pull shaft PShas been mostly broken away in FIG. 1, both at its top and where itconnects to the chuck SC. In growing the ingot I, the pulling mechanismPM lowers the seed crystal C until it contacts the surface of the moltensource material M. Once the seed crystal C begins to melt, the pullingmechanism PM slowly raises the seed crystal C up through the growthchamber GC and pull chamber PC to grow the monocrystalline ingot I. Thespeed at which the pulling mechanism PM rotates the seed crystal C andthe speed at which the pulling mechanism raises the seed crystal SC arecontrolled by the external control system (not shown). The generalconstruction and operation of the crystal puller CP, except to theextent explained more fully below, is conventional and known by those ofordinary skill in the art. Moreover, it will be understood that thecrystal puller CP can have other configurations without departing fromthe scope of the present invention.

As shown in FIGS. 1 and 3, the melter assembly 1 comprises a meltingvessel, generally designated 5, and a docking collar 9 extending downfrom the bottom of the melting vessel for connection to the lowerhousing LH of the crystal puller CP. The melting vessel 5 includes anannular floor 13, projecting up from which is a tubular side wall,generally designated 15, formed by spaced inner and outer wall members17, 19. A top wall, generally designated 23, of the melting vessel 5 isgenerally dome-shaped, and is formed by inner and outer spaced wallmembers 25, 27. The spaced wall members of the tubular wall 15 and topwall 23 define a cavity 31 through which cooling water may becirculated. A central feed portal 35 in the top wall 23 of vessel 5 hasa sleeve 39 that receives a feed tube 43 of the gravity feeder GF. Thecentral feed portal 35 has a vacuum seal 47 to seal the atmosphere inthe melter assembly 1 and gravity feeder GF. The atmospheres of thefeeder and melter assembly have substantially the same pressure andcomposition as the atmosphere in the crystal puller CP. The fit betweenthe feed tube 43 and the sleeve 39 should be a loose fit with a radialclearance of approximately 13 mm (½ inch) to contain melt splatter anddust in the melter assembly 1. The top wall 23 and tubular wall 15 ofthe melting vessel 5 have respective mating flanges 51, 53 which receivefasteners 57 to connect the top and tubular wall together. The top wall23 can be removed to permit access to the interior of the melting vessel5. The melting vessel has a side access port 59 that allows entry ofelectrical power lines (60, 63) and cooling piping (not shown) into thevessel. The side access port 59 has vacuum seals (not shown) thatsurround the piping and electrical lines 60, 63 that are routed throughthe port so that the pressure and composition of the atmosphere in thevessel 5 is maintained.

The melting vessel 5 contains a melting crucible, generally indicated65, made of a suitable material such as fused quartz (or fused silica)that is seated on a graphite susceptor assembly, generally indicated 69.The susceptor assembly 69 is supported by a ceramic base 73 mounted on aplatform 77 that covers an opening 81 in the annular floor 13 of themelting vessel 5. The tight seal between the platform 77 and the annularfloor 13 prevents melt splatter and dust from passing from the meltingvessel 5 through the docking collar 9 and into the crystal puller CP.The melting crucible 65 has a nozzle, generally indicated 85, whichdepends from a main body 89 of the crucible and allows for the flow ofmolten silicon from the melting crucible to the crystal growing crucibleCC in the crystal puller CP. The susceptor assembly 69 conforms to andsurrounds both the main body 89 and the nozzle 85 of the meltingcrucible 65. The melter assembly 1 has an upper induction coil 95 aroundan upper section of the susceptor assembly 69 and crucible 65 and alower induction coil 99 around a lower portion of the susceptor and thecrucible nozzle 85. The upper and lower induction coils 95, 99 areconnected to a respective-power supply 100, 101 (shown schematically inFIGS. 1 and 3) that provides electric current that can flow through eachrespective coil. The upper and lower coils 95, 99 can be separatelycontrolled so that the current flowing through each coil can bemonitored and adjusted without regard for the current flowing throughthe other coil. The arrangement of the two coils 95, 99 around the upperand lower sections of the melting crucible 65 and susceptor 69 allowsfor independent temperature control in the upper and lower sections ofthe melting crucible.

With reference to FIGS. 1 and 4, the main body 89 of the meltingcrucible 65 is generally cylindrical and has an open top 103, and aconical bottom wall 107. The nozzle 85 depends from the conical bottomwall 107 and is coaxial with the main body 89. In one embodiment, theinner surface of the bottom wall 107 of the crucible 65 has a slope inthe range of about 1 degree to about 60 degrees, more preferably about10 degrees, to facilitate the flow of molten silicon into the nozzle 85.In the illustrated embodiment, the main body 89 of the crucible 65including the conical bottom wall 107 is fabricated as one piece, andthe bottom nozzle 85 is fabricated from a quartz tube fused to thebottom wall of the crucible. Other constructions may be used within thescope of the present invention.

The nozzle 85 is configured to produce flow from the melting vessel 5that is generally directed along a path aligned with the centerline ofthe nozzle. It is important that the liquid silicon flowing from thecrucible 65 is a directed stream to reduce splashing and spraying of thecomponents of the crystal puller CP. Splashing and spraying of liquidsilicon is not desired in the hot zone of the crystal puller CP becauseof possible damage to the hot zone parts (e.g., heat shield HS, crucibleGC, susceptor S, etc.) and the possible creation of loose particles ofsilicon that could fall back into the crystal growing crucible andjeopardize the quality of the crystal. As best shown in FIG. 9, themelting crucible nozzle 85 includes a first larger diameter portion 109in the bottom wall 107 of the crucible 65, a second portion 111 having asmaller diameter and forming an orifice of the nozzle downstream fromthe larger diameter portion, and a third portion 115 having anintermediate size diameter downstream of the orifice. As will bediscussed in more detail below, the overall length of the nozzle 85 andthe corresponding lengths and diameters of the first, second, and thirdportions 109, 111, 115 are sized to maintain a stream of molten siliconthat flows from the crucible 65 at an optimum flow rate and avoidspremature pouring. Preferably the stream of molten silicon flowing fromthe crucible 65 is a coherent unbroken stream of liquid but it isunderstood that the stream may include droplets flowing from thecrucible 65 that are generally directed along the centerline of thenozzle 85. It is understood that the flow of molten silicon from thenozzle 85 may deviate from the centerline of the nozzle and still begenerally directed along the path of the nozzle without departing fromthe meaning of “generally directed”. In one embodiment, the flow ofmolten silicon can deviate from the centerline of the nozzle 85 by amaximum distance of approximately 40 mm (1.6 inches) and still be withinthe meaning of “generally directed”.

As shown in FIGS. 4-8, the susceptor assembly 69 has three parts: agenerally cylindric upper portion (crucible support), generallyindicated 121, to support the melting crucible 65, a conical one-pieceintermediate portion (melt-in susceptor) 125 for supporting the conicalbottom wall 107 of the crucible 65 and initiating melting, and a loweroutlet portion (nozzle susceptor) 129 that receives the nozzle 85 of themelting crucible 65. The upper portion 121 and the lower outlet portion129 are spaced apart from the intermediate portion 125 by respectiveannular gaps 130, 131. In the illustrated embodiment, the upper portion121 has four radial sections, generally designated 123 (FIG. 6), eachseparated by a radial gap 132. In one embodiment, the annular gaps 130,131 and the radial gaps 132 range from approximately 3 mm (⅛ inch) toapproximately 6 mm (¼ inch). The gaps 130, 131, 132 isolate the currentsinduced in the susceptor assembly 69 by the upper coil 95 and lower coil99 so that the lower outlet portion 129 is heated by current inducedfrom the lower coil, the intermediate portion 125 is heated by currentinduced from the upper coil, and the upper portion 121 is not heated bycurrent induced from either coil. It is understood that the upper coil95 and lower coil 99 may be assembled in a ceramic body (not shown) thatcontains graphite fiber insulation (not shown) in the annular gaps 130,131 between respective portion 121, 125, and 129 of the susceptorassembly and graphite insulation (not shown) between the radial gaps 132between respective sections 123 of the upper portion 121 of thesusceptor assembly. It is understood that the insulation may includeother materials such as quartz sand, ceramic refractory fiber, vitriousrefractory fiber, or any other high-temperature thermally andelectrically insulating material.

As shown in FIGS. 5 and 6, the upper body 121 has a series of fingers133 that are spaced apart in a circumferential direction to form gaps137 in the upper body. The gaps 137 between the fingers 133 and the gaps132 between the sections 123 of the upper portion 121 prevent the upperportion from electrically coupling to the upper induction coil 95 andprevent an induction current from flowing in the fingers of thesusceptor assembly 69. In the illustrated embodiment, sixteen fingers133 are shown but it is understood that the susceptor 69 may have otherdesigns without departing from the scope of this invention. Each of theradial gaps 132 divide one of the fingers 133 in half and the gaps arespaced apart approximately 90 degrees from each other. The gaps 137between the fingers 133 preferably range from approximately 3 mm (⅛inch) to approximately 6 mm (¼ inch). Because no current is induced inthe fingers 133 of the susceptor assembly 69, the upper portion-121 ofthe susceptor does not generate a large amount of resistance heating.Rather, the resistance heating of the lower portion of the susceptorassembly 69 from the induced current from the upper and lower coils 95,99 heats the melting crucible 65 and the solid polysilicon in the lowerportion of the crucible. The fingers 133 are not subjected to inductionheating and are heated only by radiation and conduction when the siliconin the crucible is heated by the upper coil 95.

As shown in FIGS. 5-8, the conical intermediate portion 125 and loweroutlet portion 129 of the susceptor assembly 69 are solid constructionso that the upper and lower induction coils 95, 99 are electricallycoupled to a respective portion of the susceptor in these regions. Thegap 131 between the intermediate portion 125 and the outlet portion 129electrically isolates the two portions of the susceptor assembly 69 sothat the upper coil 95 does not induce current in the lower outletportion and the lower coil 99 does not induce current in theintermediate portion. The electrical current supplied to the upper coil95 produces an alternating magnetic field which causes electricalcurrent to flow in the intermediate-portion 125 of the susceptorassembly 69 below the fingers 133. The electrical current supplied tothe lower coil 99 produces an alternating magnetic field which causeselectrical current to flow in the lower outlet portion 129 of thesusceptor 69. The induced electrical current in the susceptor assembly69 flows through the susceptor in the opposite direction as the currentflow in the coils 95, 99 and causes resistance heating in the susceptor.The induction heating of the solid portion of the graphite susceptorassembly 69 causes radiant and conductive heating of the bottom wall 107of the crucible 65 so that the solid polysilicon resting on the bottomwall of the crucible melts.

After the resistance and conductive heating has melted and the bottomportion of the solid polysilicon in the crucible 65, the magnetic fieldof the upper coil 95 induces a current in the electrically conductiveliquid silicon that further heats the liquid silicon and increases therate of melting of the remaining solid polysilicon in the crucible byradiation and conduction heating from the liquid silicon. In oneembodiment, the frequency of the alternating current supplied to thecoils 95, 99 ranges from approximately 3 kHz to approximately 15 kHz andthe magnitude of the power ranges from approximately 15 kW toapproximately 160 kW. In one embodiment, the electrical current in theupper coil 95 has a frequency of approximately 10 kHz and power ofapproximately 160 kW, and the electrical current in the lower coil 99has a frequency of approximately 3 kHz and power of approximately 15 kW.It will be understood that each power supply 100, 101 may comprise apower converter, a motor-generator, a pulse-width modulator inverter, orany other means for supplying alternating current to the coils.

In one embodiment, the cylindric body 121 of the susceptor assembly 69may have a height of approximately 38 cm (15 inches), an inside diameterof approximately 20 cm (8 inches), and a wall thickness of approximately13 mm ( 1/2 inch). The bottom wall of the conical portion 125 of thesusceptor 69 may be angled to correspond with the angled bottom wall 107of the crucible 65 and may range from approximately 1 degree toapproximately 60 degrees (more preferably about 10 degrees) withoutdeparting from the scope of this invention. The lower outlet portion 129of the susceptor 69 may extend from the bottom wall of the conicalportion 125 of the susceptor by approximately 16.5 cm to 30 cm (6.5inches to approximately 12 inches). It will be understood that thedimensions are exemplary only and the crucible 65 and susceptor 69 mayhave other dimensions without departing from the scope of presentinvention.

The current through the upper coil 95 may be varied to control thetemperature of the melt in the crucible 65 and the current through thelower coil 99 may be varied to control the temperature of the meltpassing through the crucible nozzle 85. For example, the temperature ofthe melt in the melting crucible 65 must be above the meltingtemperature of silicon (1414 degrees C.) by a sufficient amount so thatthe melt being poured into the crystal growing crucible CC of thecrystal puller CP remains above the melting temperature of siliconduring the free fall from the melting crucible to the crystal growingcrucible. Based on a free fall height F (FIG. 3) of approximately 5 feet(1.5 meters), the silicon liquid passing through the nozzle 85 shouldhave at least 20 degrees of superheat (corresponding to a minimumtemperature of approximately 1434 degrees C.) so that the silicon flowstream remains liquid for the entire distance between the largerdiameter portion 109 of the melting crucible nozzle and the surface ofthe melt M in the crystal growing crucible CC of the crystal puller CP.Also, the temperature of the molten silicon passing through the quartznozzle 85 of the melting crucible 65 should not exceed approximately1465 degrees C. in order to prevent excessive ablation of the nozzle asa result of the flow of superheated liquid silicon through the nozzle.If the molten silicon passing through the nozzle 85 is allowed to flowat a temperature above 1465 degrees C, the first and second nozzleportions 111, 115 will prematurely become too large and the mass flowrate of liquid silicon will exceed the desired melting rate of thepolysilicon. The mass flow rate of liquid silicon through the nozzle 85should be less than or equal to the desired melting rate of thepolysilicon in the crucible 65 so that height of the melt in thecrucible remains generally constant during the filling of the crystalgrowing crucible CC. If the flow rate from the crucible 65 exceeds themelting rate of polysilicon, the melt height in the crucible 65 willdrop during filling of the crystal growing crucible CC causing areduction in the static pressure at the nozzle 85. If the staticpressure at the nozzle 85 drops below a predetermined level, the streamof liquid silicon passing through the nozzle will become undirected(i.e., no longer generally directed along the centerline of the nozzle)resulting in excessive splashing and spraying at the crystal growthcrucible CC.

To obtain a reduction in melt preparation time needed to achieve animprovement in crystal growth productivity of at least approximately 10%to 35%, the melter assembly 1 should deliver liquid silicon at apreferred mass flow rate in the range of approximately 50 kg/hr toapproximately 140 kg/hr. Through experimentation, the more preferablemass flow rate of liquid silicon from the melter assembly 1 has beendetermined to be approximately 85 kg/hr. The orifice 111 is sized tohave an initial diameter that provides the optimum mass flow rate ofliquid silicon of approximately 85 kg/hr based on an initial melt heightH (FIG. 14) above the bottom outlet 85 of the crucible 65 ofapproximately 250 mm (10 inches). It will be understood that the orificediameter and melt height H above the orifice 111 will vary based on thediameter and height of the melting crucible 65 as well as the desiredmass flow rate from the melter assembly 1.

Referring now to FIG. 9, the orifice 11 in the nozzle 85 of the crucible65 has an initial diameter D1 of approximately 2.3 mm (0.09 inch)corresponding to an initial melt height H (FIG. 14) of approximately 250mm (10 inches) and a liquid silicon flow rate of approximately 85 kg/hr.It will be understood that as the melter assembly 1 is used in fillingsuccessive crystal pullers CP with molten silicon, the orifice diameterD1 will increase due to ablation of the wall thickness of the cruciblenozzle 85 by the flow of liquid silicon therethrough. Accordingly, theinitial melt height H above the orifice 111 is preferably reduced as thediameter of the orifice increases to maintain an optimum mass flow rateof liquid silicon. The mass flow rate of liquid silicon through theorifice 111 is determined by application of Torricelli's Law that statesthe velocity (v) of liquid leaving an orifice equals the square root oftwice the height above the orifice (h) times a gravitational constant(g) orv=(2×h×g)^(1/2)

-   -   where v=velocity of liquid passing through orifice,    -   where h=melt height H, and    -   g=gravitational constant.

The mass flow rate of the liquid flowing through the orifice 111 is thencalculated by multiplying the velocity of liquid from the above equationby the area of the orifice and the density of the liquid.

The relative size of the orifice 111 of the nozzle 85 is monitored byregulating the melt H that is required to maintain the desired mass flowrate of liquid silicon from the crucible 65 during successive operationsof the melter assembly 1. When the orifice 111 has ablated such that thedesired mass flow rate of liquid silicon exceeds the attainable meltingrate, the crucible 65 is replaced. In one embodiment, the orificediameter D1 may be allowed to increase to 3.0 mm (0.12 inch) before themelting crucible 65 is replaced. To maintain a liquid silicon flow rateof 85 kg/hr at an orifice diameter D1 of about 3.0 mm (0.12 inch), themelt height H must be lowered to approximately 177 mm (7.0 inches).

To avoid excessive splashing of liquid silicon as it impinges the meltpool M in the crystal growing crucible CC, a directed stream of liquidsilicon must flow from the crucible nozzle 85 to the melt in the crystalgrowing crucible. Based on the results of testing various nozzle designsat a liquid silicon flow rate of approximately 85 kg/hr, the preferreddesign of the melting crucible nozzle 85 was determined to include aninitial diameter D2 for the first portion 109 of approximately 6 to 14mm (0.24 to 0.55 inch), or more preferably approximately 10 mm (0.39inch), and a length L1 of the first portion of approximately 50 to 60 mm(1.96 to 2.4 inches), or more preferably approximately 55 mm (2.16inches). The orifice 111 of the nozzle 85 may have an initial innerdiameter D1 ranging from approximately 2.1 to 2.3 mm (0.08 to 0.09inch), or more preferably approximately 2.2 mm (0.087 inch), and alength L2 of approximately 8 to 12 mm (0.39 to 0.47 inch), or morepreferably approximately 10 mm (0.39 inch). The third portion 115 mayhave an inside diameter D3 ranging from approximately 3 to 8 mm (0.12 to0.31 inch), more preferably approximately 3 mm (0.12 inch), and a lengthL3 ranging from approximately 76 mm to 210 mm (3.0 inches to 8.3inches), more preferably approximately 118 mm (4.6 inches). It wasdetermined, that the preferred length L3 of the third portion 115 of thenozzle 85 can be calculated in millimeters from the following formula:L3=50+(50×D3/D1).The above formula can be used to determine the preferred length L3 ofthe third portion 115 of the nozzle 85 for a mass flow of liquid siliconranging from approximately 50 kg/hr to approximately 140 kg/hr. It willbe appreciated that the preferred configuration of the first, second,and third portions 109, 111, 115 of the nozzle 85 depends upon variousfactors, including the desired mass flow rate of liquid silicon.Accordingly, the present invention is not limited to specific dimensionsof the nozzle 85 listed herein.

As shown in FIG. 3, a splash guard assembly, generally designated 151,is supported by the docking collar 9 and comprises a splash guard 155connected to four pulleys 159, 161 (only two of which are shown) by fourcables 163, 165 (only two of which are shown) extending up from theguard. The pulleys 159, 161 are connected to a shaft 169 having a handle173 that is accessible from outside of the docking collar 9. The handle173 may be manually turned to rotate the shaft 169 and raise or lowerthe splash guard 155 during operation of the melter assembly 1. Thesplash guard 155 has an inverted bowl shape with an open top and bottomto allow molten silicon to pass through the splash guard. The splashguard 155 may be made from fused quartz, fused silica, silicon-carbidecoated graphite, or other suitable material. The splash guard 155 issized to fit inside the heat shield HS of the crystal puller CP so thatthe guard may be lowered close to the surface of the melt M in thecrystal growth crucible CC as shown in phantom in FIG. 3. The splashguard assembly 151 protects the heat shield HS and other hot zonecomponents from splashing and spraying of liquid silicon in the crystalpuller CP during the transfer of liquid between the melter assembly 1and the crystal growth crucible CC.

In use, the melter assembly 1 of the present invention efficiently meltsand pours polysilicon that is received from the gravity feeder GF. Themelter assembly 1 has a modular design that is designed to be easilyconnected to the crystal puller CP to recharge the crystal growingcrucible CC with liquid silicon and then removed from the crystal pullerCP for installation of the upper pull chamber PC for production ofsilicon ingots I. The portable design of the melter assembly 1 allows asignificant cost savings by requiring only a single melter assembly thatmay service multiple crystal pullers CP (e.g., eight).

With an empty melting crucible 65, cooling water (not shown), electricalpower (not shown), and purge gas (not shown) are connected to the melterassembly 1. Next, the gate valve GV in the lower housing LH (FIG. 2) isclosed to isolate the lower housing and the upper housing UH of thecrystal puller CP is removed from the lower housing and replaced withthe melter assembly 1 as illustrated in FIG. 3. After tightening theflanged connection FC between the docking collar 9 of the melterassembly 1 and the lower housing LH, the melter assembly is connected tothe polysilicon feeder GF. After connecting the feeder GF to the melterassembly 1, the air in the melter assembly is removed by vacuum pumpingand replaced with an inert gas (e.g., argon gas) so that the pressure inthe melter assembly is approximately that of the crystal puller CP(approximately 10-30 torr). After the pressure in the melter assembly 1has been raised, the gate valve GV is opened to allow the atmosphere inthe crystal puller CP to permeate the melter assembly 1.

As shown in FIG. 10, granular polysilicon GP is added to the meltercrucible 65 to provide an initial charge of approximately 4 to 7 kg (fora melting crucible-having an 20 cm (8 inch) inside diameter) with theoptimum mass for the initial charge of granular polysilicon forinitiating melting in an 20 cm (8 inch) melting crucible 65 being about5 kg. The power from the power supply 100 is turned on to provide 10 kWof alternating current to the upper coil 95 around the susceptor 69. Asshown in FIG. 11, the initial charge of polysilicon GP in the cruciblebegins to melt due to the heat from the melt-in susceptor 125 and formmolten silicon LS generally near the bottom of the initial charge ofgranular polysilicon and above an unmelted plug of solid polysilicon GPin the nozzle 85 at the bottom of the crucible 65. At the initialmelting stage of FIG. 11, a top layer of unmelted polysilicon GP isseparated from the molten silicon LS by an intermediate layer SL thatincludes a slush mixture of liquid and solid polysilicon. Theintermediate layer SL is separated from the molten silicon by an annularvoid AV adjacent the conical wall 107 of the melting crucible 65. As thegranular polysilicon GP melts the difference in density between theunmelted granular polysilicon and the liquid silicon (LS) causes thevoid AV in the melting crucible 65. As shown in FIG. 11A, melting in thecrucible 65 continues with approximately 12-16 kW of power supplied tothe coil 95 so that all of the initial granular polysilicon GP is meltedto molten silicon LS except for a crust CL of solid polysilicon spacedabove the top of the molten silicon. The crust CL holds in the heat ofthe molten silicon LS and prevents the liquid silicon from splashing outof the crucible 65 during movement in the crucible by the inductionforces from the coil 95. At this stage in the melting process, a voidspace VS separates the crust CL from the liquid silicon LS in thecrucible. As shown in FIG. 12, an opening OG is formed in the crust bypulsing the upper coil 95 by increasing the power to the coil toapproximately 60-70 kW for short time intervals (e.g., 20-30 seconds) sothat the level of the liquid silicon LS is raised by the inductionforces from the coil to contact and melt through the crust CL. Thepulsing of the coil 95 for short time intervals continues forapproximately 2 to 5 minutes until the crust CL is melted through.

After melting through the crust CL, additional granular polysilicon GPis added to the crucible 65 (FIG. 13). At this point in the meltingprocess, the solid plug OP extending into the first portion 109 of thenozzle 85 preventing molten silicon LS from exiting the crucible 65allows the melt height H to be raised to the desired level before thecrucible is drained. As shown in FIG. 13A, granular polysilicon GP isadded at approximately the maximum rate (e.g., about 70 to 85 kg/hr) tobegin raising the melt height and continue melting away the remnants ofthe crust layer CL. At this stage, the power to the coil is increased toapproximately 160 kW for a maximum flow rate of granular polysilicon GPof approximately 70-85 kg/hr. In the illustrated embodiment the meltheight H (FIG. 13A) at this stage of the melting is approximately 38 mmto 89 mm (1.5 inches to 3.5 inches).

After melting the initial charge of granular polysilicon GP, additionalgranular polysilicon is added to the crucible (FIGS. 13 and 13A) andmelted until the melt height H (FIG. 14) is raised to approximately 250mm (10 inches). Next, the splash guard 155 is lowered into the hot zoneof the crystal puller CP and positioned in the crystal growing crucibleCC generally near the surface of the silicon melt M (shown in phantom inFIG. 3). After lowering the splash guard 155 (FIG. 3), power is suppliedto the lower coil 99 from the power supply 101 to raise the temperatureof the bottom wall 107 of the crucible and begin melting the solid plugOP in the crucible nozzle 85 (FIG. 14). At this stage, the powersupplied to the upper coil 95 is reduced to prevent the liquid siliconLS from overheating. When the solid plug OP has been melted by theinduction current in the lower coil 99, molten silicon LS begins to flowfrom the nozzle 85 and passes through the first, second and thirdportions 109, 111, 115 in a directed flowstream (FIG. 15) generallyaligned with the central axis of the nozzle. When liquid silicon LSbegins to drain from the crucible 65 the feeder GF should be set to thetarget polysilicon feed rate of about 85 kg/hr. During the flow ofmolten silicon LS from the crucible 65, the melt height H of the liquidsilicon in the crucible should be monitored and the feed rate ofgranular polysilicon GP adjusted accordingly to maintain the height H atthe optimum level corresponding to the diameter D1 of the orifice 111.If the melt height H rises above the optimum value, the polysilicon feedrate should be reduced and if the melt height falls below the optimumvalue, the feed rate should be increased. While the molten silicon LS isdraining from the crucible 65, the power to the upper and lower coils95, 99 should be adjusted to maintain the optimum amount of superheat(e.g., at least approximately 20 degrees C.) of the liquid siliconexiting the crucible nozzle 85.

Once the target amount of granular polysilicon GP has been deliveredfrom the feeder GF, the granular polysilicon feed is turned off andpower to the upper coil 95 reduced to approximately 12 kW. At thisstage, the crucible 65 is drained (FIG. 16). If needed, purge gas (e.g.argon gas) can be added to the melter assembly to increase the pressurein the assembly and force any remaining silicon out of the cruciblenozzle at an appropriate rate. As the melt height H in the crucible 65drops, the mass flow rate of liquid silicon LS passing through theoutlet 85 of the crucible is reduced. The purge gas can be used topressurize the remaining liquid silicon LS in the crucible 65 so thatthe liquid silicon flows out the nozzle 85 of the crucible at asufficient velocity to maintain a flow stream generally directed alongthe central axis of the nozzle. It has been determined thatapproximately 2 torr (0.04 psi) of pressure differential between themelter assembly 1 and the crystal puller CP is preferable forapproximately every 10 mm (0.4 inch) reduction in melt height H tomaintain the desired mass flow rate of liquid silicon LS. Aftercompletely draining the melter of all liquid silicon LS, power to theupper and lower coils 95, 99 may be turned off and the splash guard 155raised into the docking collar 9. The gate valve of the lower housing LHis closed to isolate the growth chamber GC. Next, the melter assembly 1is filled with air at ambient pressure in order to oxidize the siliconoxide present in the melter assembly and prevent uncontrolled burning ofsilicon oxide upon disconnection of the melter assembly from the crystalpuller. After filling the melter assembly 1 with air, the assembly canbe disconnected from the crystal puller CP and stored between meltingand draining cycles or the assembly may be immediately prepped forrecharging another crystal puller. After removing the melter assembly 1,the upper housing UH of the crystal puller CP can be replaced and thepuller prepared to produce silicon ingots I from the melt M.

The operation of the melter assembly 1 of the present invention resultsin a total melting and fill time of the polysilicon source material inthe crystal growing crucible CC of the crystal puller CP ofapproximately five to five and one-half hours. The fill and melting timeof the conventional melting pool preparation in the crystal growingcrucible CC of the crystal puller CP takes approximately 18 hours for a250 kg (551 lbs) charge. The significant time savings by using themelter assembly 1 of the present invention significantly increases theingot I production of each crystal puller CP filled by the melterassembly 1. Also, because the melting of polysilicon is isolated in themelting assembly 1 and does not take place in the hot zone of thecrystal puller CP, less expensive, non-dehydrogenated polysilicon sourcematerial may be used because the splashing and spraying from addingpolysilicon having a higher amount of hydrogen is contained in themelter assembly.

As shown in FIG. 29, the melter assembly 1 of the present invention maybe used to service multiple crystal forming apparatus (e.g., crystalpullers CP). The melter assembly 1 may be operated as discussed above todeliver a charge of molten source material LS to the crystal puller CPand then moved by a lifting mechanism (not shown) and installed on anadjacent crystal puller. Four crystal pullers CP are shown positionedfor service by the melter assembly 1, but it is understood that themelter assembly could service more or less than four crystal pullerswithout departing from the scope of this invention.

The melter assembly 1 is first positioned relative to the first crystalpuller CP and is operated as described above to deliver molten siliconLS to the crucible CC of the first crystal puller. As described above,the upper and lower induction coils 95, 99 of the heater are operated tomelt source material in the melting crucible 65 and deliver a stream ofmolten source material that flows through the nozzle 85 of the melterassembly 1 to the main crucible CC of the first crystal puller CP. Afterservicing the first crystal puller CP, the upper housing UH of thesecond crystal puller that encloses the pull chamber PC is removed andthe melter assembly 1 is positioned above the gate valve GV isolatingthe lower housing LH of the second puller. The melter assembly 1 may bepositioned relative to the first and second crystal pullers CP by acrane (not shown) or other lifting mechanism. The crane may be used toraise the melter assembly 1 above the first crystal puller CP so thatthe melter assembly may be transferred to a position above the secondcrystal puller. After positioning the melter assembly 1 relative to thesecond crystal puller CP, the melter assembly is connected to the lowerhousing LH of the second crystal puller. After connecting the melterassembly to the crystal puller, the melter assembly is operated todeliver a stream of molten source material to the second crystal formingapparatus. It is understood that the method of servicing multiplecrystal forming apparatus could be applied to any embodiment of crystalforming apparatus described herein or any other crystal formingapparatus without departing from the scope of this invention. Further,it is understood that after completion of the melt and fill cycle, themelter assembly 1 may remain stationary and the crystal puller CP may bemoved relative to the melter assembly and replaced with the secondcrystal puller. It is understood that the crystal puller may be used toservice more or less than four crystal pullers CP that are illustratedin FIG. 29.

As shown in FIG. 17, another melting assembly is shown, generallyindicated 201, that is substantially similar to the melting assembly 1of the first embodiment. The melting assembly 201 of FIG. 17 includes amelting crucible 203 with a closed top wall 207. The closed top wall 207of the melting crucible 203 contains splatter that may occur-whengranular polysilicon GP is added to the molten silicon LS in thecrucible. Also, the top wall 207 contains polysilicon dust that iscreated when the granular polysilicon GP is loaded into the meltingcrucible 203.

FIG. 18 shows yet another embodiment of the melting assembly, generallyindicated 215, that includes an upper coil 219, an intermediate coil221, and a lower coil 223 for supplying induction current to melt thesolid polysilicon GP. This design also includes a melting crucible 227with a closed top wall 231 as in the previous embodiment, but it will beunderstood that this design may be incorporated to the open-top crucibledesign without departing from the scope of this invention. Thethree-coil configuration of this embodiment allows temperatureregulation of the melt by control of the current supplied to the uppercoil 219, intermediate coil 221, and lower coil 223. Each of the coils219, 221, 233 is separately connected to its own power supply forindependent control of the current passing through each coil. The amountof heating power generated by each of the three coils 219, 221, 223 maybe independently regulated.

The addition of the separate intermediate coil 221 of this design allowsthe heat of the silicon melt in the middle portion of the crucible 227to be independently regulated by adjusting the current flowing throughthe intermediate coil. The separate lower coil 223 of this design allowsthe heat of the silicon flowing through the outlet 235 to beindependently regulated by adjusting the frequency of the currentflowing through the lower coil. Also, the lower coil 223 can be used tocreate a magnetic field in the outlet 235 that restricts the flow ofsilicon to a smaller diameter than the inner diameter of the outlet 235.The reduction of flow stream diameter passing through the outlet 235allows further control of melt flow rate from the crucible 227 andreduces the rate of ablation of the outlet as a result of the reducedcontact between the liquid silicon passing through the outlet and theinner surface of the outlet.

FIG. 19 shows a further embodiment of the melter assembly, generallyindicated 251, that includes a crucible 255 having a closed top wall 259with a polysilicon feed inlet 263 having two 45-degree bends 265, 267for accommodating the outlet of the gravity feeder GF. This designcontains both polysilicon dust that is generated from transferring thesolid polysilicon to the crucible and the melt splatter that frequentlyoccurs when the polysilicon impinges the melt in the crucible. The twobends create a tortuous path inhibiting splatter polysilicon fromreaching the gravity feeder. Also, the two bends accommodate a gravityfeeder that is offset from the centerline of the melter assembly so thatthe feeder can be connected to the melter assembly from a differentposition of the feeder.

FIGS. 20 and 21 show yet another embodiment of the melter assembly,generally indicated 301, that includes a crucible, generally indicated305, having-multiple chambers. The crucible 305 comprises an outer bowl309 having a bottom nozzle 311, a filter cylinder 315 welded to theouter bowl and having ports 317 (only two of which are shown) for theinflow of molten silicon LS, and an inner cylinder weir 321 sized tocorrespond with the size of the bottom nozzle. The weir 321 is shorterthan the bowl 309 and filter cylinder 315 and surrounds the nozzle 311so that liquid silicon LS inside of the cylindrical weir exits thenozzle of the crucible 305. The melting crucible 305 of this design hasa melting chamber 325 defined by the space between the bowl 309 and thefilter cylinder 315, a control chamber 329 between the filter cylinderand the weir 321, and a drain chamber 333 defined by the space insidethe cylindrical weir 321.

The nozzle 311 may be constructed as describe above for nozzle 85 andmay be used in combination with a resistance heated melt flow guide 337.The flow guide 337 controls the flow of liquid silicon from the meltingcrucible 305 by directing the flow from the melting crucible along theouter surface of the flow guide and into the growth crucible CC. In theembodiment of FIGS. 20 and 21A, the melt flow guide 337 is in the formof a fused quartz tube that is received through the nozzle 311 of theouter bowl 309 and is axially aligned with the drain chamber 333. Themelt flow guide 337 has a heating element 339 on'the inside of the tubethat maintains the flow guide at a temperature approximately equal to orslightly above the melting point of silicon (approximately 1414 degreesC.). As shown in FIG. 21A, the flow guide 337 extends down into thecrystal growing crucible CC of the crystal puller CP (FIG. 2) andfacilitates the flow of molten silicon from the melting assembly 301 tothe crystal puller so that splashing and spraying in the crystal growingcrucible is reduced. The flow guide 337 may be made of any suitablematerial (e.g., fused silica, fused quartz, silicon-carbide coatedgraphite, etc.) without departing from the scope of this invention. Theheating element 339 may be an electric resistance heating element foruse with a molybdenum, tungsten, or graphite resistance heater.Alternatively the flow guide 337 may be heated by induction heating witha molybdenum, tungsten, or graphite susceptor.

FIG. 21B shows a modified version of the flow guide 341 in the form of atube that receives the nozzle 311 of the melting crucible 305. The flowguide 341 has a heating element 343 on the outside of the tube so thatmolten silicon LS flows from the nozzle 311 into the tube. As with theprevious version, the flow guide 341 directs the flow of molten siliconLS from the melting crucible 305 to the growth crucible CC of thecrystal puller CP (FIG. 2). As with the previous embodiment, the flowguide 341 may be heated by resistance heating, induction heating, or anyother heating method.

FIG. 21C shows another version of the flow guide 345 that receives thenozzle 311 of the melting crucible 305. The flow guide 345 is sized suchthat a coherent stream of molten silicon LS flowing from the nozzle doesnot contact the wall of the flow guide. Since the molten silicon doesnot contact the wall of the flow guide 345, the guide does not have aheating element as with the previous versions. The flow guide 345prevents liquid silicon LS from the growing crucible CC from contactingthe hot zone components of the crystal puller when the flow stream fromthe nozzle 311 enters the pool of liquid silicon in the growingcrucible.

In the design of FIGS. 20 and 21, granular polysilicon GP is added tothe melt chamber 325 and the polysilicon is melted in a similar manneras discussed above for the first embodiment (i.e., induction heating byinduction coils 341 around the crucible 305). As the granularpolysilicon GP melts in the melting chamber 325, molten silicon LS flowsthrough the ports 317 in the filter cylinder 315 and fills the controlchamber 329 of the crucible 305. Once the level of molten silicon LS inthe control chamber 329 reaches the height of the cylindrical weir 321,molten silicon flows over the top of the weir and begins to exit thenozzle 311 of the crucible 305. The molten silicon LS contacts the flowguide 337 and flows down to the crystal growing crucible CC at a slow,controlled speed and in a direction generally axially aligned with thenozzle 311 to minimize disturbances on the surface of the melt M in thecrucible. In this way, the melter assembly 301 supplies a controlledflow of molten silicon LS to the crystal growth crucible CC in a mannerthat reduces splashing of liquid silicon onto the hot zone parts of thecrystal puller CP. The ports 317 in the filter cylinder 315 are locatedbelow the surface of the liquid silicon LS so that any unmelted siliconthat may be floating in the liquid silicon is not allowed to pass fromthe melting chamber 325 to the control chamber 329 of the assembly 301.

FIG. 22 shows a melter assembly, generally designated 351 similar to theprevious embodiment except that the melter assembly of this embodimentincludes a melting crucible 355 having only a single internal chamber357. The crucible 355 is surrounded by a melting induction coil 361 formelting polysilicon in the crucible. The outlet 363 of the crucible 355receives a melt guide 367 and is surrounded by a lower induction coil371 for the flow of alternating electrical current from a power supply(not shown). The lower induction coil 371 (also referred to as alevitation valve) serves as a nozzle valve that controls the flow ofliquid silicon LS out of the crucible 355. When the current is flowingthrough the lower induction coil 371, the magnetic field induced in theflow path of liquid silicon LS is strong enough to block the flow ofliquid-silicon from the crucible 355. The flow of liquid silicon LS fromthe crucible 355 is controlled by turning on and off the currentsupplied to the lower induction coil 371.

FIG. 23 shows a melter assembly of another embodiment, generallyindicated 381, that is similar to the previous embodiments except themelter assembly is designed to transfer liquid silicon LS into thecrystal growing crucible CC by pouring liquid silicon over the top wall383 of the melting crucible 385. The melter assembly 381 includes aheater 389 that heats the melting crucible 385 to melt solid polysiliconin a controlled environment separate from the crystal puller CP. Theheater 389 may include induction coils as described above, resistanceheaters, or any other suitable heater without departing from the scopeof this invention. After the solid polysilicon has been melted, themelting crucible 385 is positioned above the crystal growth crucible CCand tilted so that liquid silicon LS is poured over the top wall of thecrucible filling the crystal growth crucible. The crystal growthcrucible CC may be filled continuously during the operation of thecrystal puller or may be filled by a batch filling process at the end ofa crystal pulling session.

FIG. 24 shows another prior art crystal forming apparatus in the form ofan Edge-defined Film Growth (EFG) crystal growing apparatus, generallyindicated GA. The particular apparatus GA illustrated is configured forgrowing hollow 8-sided polycrystalline silicon bodies (not shown) but itis understood that the apparatus may be configured for formingcrystalline bodies having other shapes. The apparatus GA has a lowerhousing LH1 enclosing a crystal growth chamber GC1 and an upper housingUH1 enclosing a pull chamber PC1. The upper housing UH1 has been mostlybroken away in FIG. 24. In the illustrated embodiment, the lower housingLH1 encloses a crucible/capillary die system including a growth crucibleGC1, a capillary die CD1, a susceptor S1, an inner heater assembly IH1,and an outer heater assembly HA1.

The crucible GC1 contains a charge of molten source material SM1 (e.g.,polycrystalline silicon) and is surrounded by a radio frequency heatingcoil HC1 for heating the source material in the crucible. The crucibleGC1 has an end face, generally indicated EF1, having a capillary gap CG1formed therein located generally near the peripheral edge of thecrucible. The capillary gap cG1 and the crucible GC1 have a shapecorresponding to the cross-sectional shape of the crystalline bodyformed by the apparatus GA. The crucible GC1 has slots ST1 formed on aninside wall of the crucible so that molten silicon SM1 may flow into thecapillary gap CG1 and rise by capillary action. In the illustratedembodiment, the apparatus GA has a seed SC1 that is octagonally shapedto correspond with the shape of the capillary gap CG1. The seed SC1 islowered into contact with the molten silicon SM1 in the capillary gapCG1 to initiate the growth sequence. As the seed SC1 rises from thecapillary die CD1, the molten silicon SM1 in the gap CG1 is drawn out ofthe die and molten silicon from the crucible GC1 rises by capillaryaction in the capillary gap to replenish the material removed from thecrucible. Reference may be made to U.S. Pat. Nos. 5,156,978, 4,647,437,4,440,728, 4,230,674 and 4,036,666, the disclosures of which areincorporated herein by reference, for additional information regardingconventional EFG crystal forming apparatus.

FIG. 25 shows the melter assembly 1 of the present invention installedon the EFG crystal forming apparatus GA1 of FIG. 24. It is understoodthat the melter assembly 1 connects to the apparatus GA1 by removing theupper housing UH1 enclosing the pull chamber PC1 and connecting thedocking collar 9 of the melter assembly to a flange FA1 on the lowerhousing of the crystal forming apparatus. The melter assembly 1 is thenoperated in a similar fashion as discussed above to fill the crucibleGC1 of the EFG crystal forming apparatus GA1 with molten silicon SM1.When the crucible GC1 of the crystal forming apparatus GA1 is filledwith molten silicon SM1, the melter assembly 1 is removed and replacedwith the upper housing UH1 for operation of the crystal formingapparatus.

FIG. 26 shows another prior art crystal forming apparatus, generallyindicated GA2, for which the melter assembly 1 of the present inventionmay be used to supply molten source material SM2. The crystal formingapparatus GA2 of FIG. 26 is a crystal pulling apparatus that uses theString Stabilized Growth (SSG) method to produce a solid crystallineribbon CR2 which is grown from the melt SM2 contained in a crucible GC2.This crystal forming apparatus GA2 produces a thin, wide sheet of largegrain polycrystalline silicon or monocrystalline silicon that issuitable for use in the production of solar cells or other semiconductordevices.

As shown in FIG. 26, the apparatus GA2 includes two spaced apart stringsSS1, SS2 that pass through the crucible GC2 and melt SM2 containedtherein. Current is induced in the melt SM2 by a DC source (not shown)electrically connected to the crucible GC2 which causes flow circulationof the melt. A hollow barrier HB2 in the crucible GC2 reduces the meltdepth from which the crystal CR2 is grown and increases the quality ofthe crystal. It is understood that solid polycrystalline silicon (notshown) may be added to the crucible GC2 via a supply tube ST2 and meltedtherein to form the melt SM2. A heating coil HC2 (FIG. 27) surrounds thecrucible GC2 for heating the solid source material and the melt SM2 inthe crucible. The crucible GC2 has a melt tube MT2 for emptying thecrucible after the melt SM2 has become impure so that the melt may bereplaced with fresh source material. As the strings SS1, SS2 are drawnupward from the crucible GC2, liquid silicon between the strings isremoved from the crucible and solidifies as it cools to form a ribbonCR2 of solid polycrystalline silicon. It is understood that theapparatus of FIG. 26 would include a lower housing LH2 (see, FIG. 27)enclosing the crucible GC2 and an upper housing (not shown) enclosing apulling apparatus for pulling the strings and the ribbon CR2. Referencemay be made to U.S. Pat. Nos. 4,689,109, 4,661,200, and 4,627,887, thedisclosures of which are incorporated herein by reference, foradditional information regarding the SSG crystal forming apparatus.

As shown in FIG. 27, the melter assembly 1 of the present invention maybe positioned in place of the upper housing above the crucible GC2 ofthe SSG crystal pulling apparatus GA2 to recharge the crucible withmolten silicon SM2. The melter assembly 1 would be operated in a similarmanner as discussed above for the previous embodiments to deliver themolten silicon SM2 to the SSG crystal forming apparatus GA2.

As shown in FIG. 28, the melter assembly 1 of the present invention maybe used to supply molten source material SM3 to a casting apparatus,generally indicated GA3, for casting solid crystalline ingots SI3. Inthe embodiment of FIG. 28, the casting apparatus GA3 is a continuouscasting apparatus having a retractable support member SP3 below a mold,generally indicated MD3. In the illustrated embodiment, the mold MD3includes a series of spaced apart crucible fingers CF3 that aresurrounded by an RF coil RC3. The RF coil RC3 induces a current in thecrucible fingers CF3 and an oppositely charged current in the moltensource material SM3 so that the molten material is repelled from thecrucible fingers and contained in the casting apparatus GA3. Coolingliquid (not shown) is circulated through the crucible fingers CF3 tohelp cool the molten source material SM3.

Solid silicon ingots SI3 are formed in the casting apparatus GA3 bylowering the support member SP3 away from the crucible fingers CF3. Asthe retractable support member SP3 is slowly lowered away from thebottom of the crucible fingers CF3, the induced current in the bottomportion of the molten material SM3 gradually decreases allowing themolten material to cool as it moves away from the crucible fingers. Theingot SI3 is cast as the support member SP3 is lowered, allowing themolten source material SM3 that is removed from the crucible fingers CF3to cool and solidify. Typically, solid source material (not shown) thatis added from the top of the apparatus GA3 is melted in the apparatus toreplenish the molten material SM3 that is removed when the retractablesupport member SP3 is lowered. As shown in FIG. 28, the melter assembly1 of the present invention can be installed above the crucible fingersCF3 to replenish the quantity of molten source material SM3 that is usedto cast the solid ingot SI3.

The melter assembly 1 may be operated in a similar manner as discussedabove to provide a directed flow of molten source material SM3 to thecasting apparatus GA3. In the illustrated embodiment, the melterassembly 1 is first positioned above the casting apparatus such thatmolten source material SM3 may be delivered to the mold MD3 formed bythe crucible fingers CF3. After granular polysilicon GP (FIG. 10) isadded to the melter assembly 1, the upper heating coil 95 in the melterassembly is operated to melt the source material in the melting crucible65. The lower heating coil 99 is operated to melt the solid plug OPabove the crucible nozzle 85 to allow molten source material LS to flowthrough the orifice 111 of the melter assembly 1 to deliver a directedstream of molten source material to the crucible GC3 of the crystalforming apparatus GA3. After the crucible GC3 of the crystal formingapparatus has been filled, the operation of the lower heating coil 99 isdiscontinued to allow the solid plug OP of solidified source material inthe melter assembly to form. The solid plug OP prevents the flow ofmolten source material LS from the melter assembly 1 while the castingot SI3 is removed from the casting apparatus GA3 or a fresh castingapparatus is positioned below the melter assembly for the production ofthe next cast polycrystalline body.

It is understood that the casting apparatus GA3 shown in FIG. 29 may bereplaced with a conventional cold crucible mold having a solid wall forbatch-casting solid ingots without departing from the scope of thisinvention. Reference is made to U.S. Pat. Nos. 4,769,107, 4,572,812, and4,175,610 which is incorporated by reference herein, for additionalinformation regarding casting processes capable of utilizing the melterassembly 1 of the present invention.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

1. A melter assembly for supplying a charge of molten source material toa crystal forming apparatus, the melter assembly comprising: a housingadapted for serial positioning above a selected crystal formingapparatus of a plurality of crystal forming apparatus; a cruciblelocated in the housing; a heater disposed relative to the crucible formelting solid source material received in the crucible; the cruciblehaving a nozzle comprising a first larger diameter portion, a secondsmaller diameter portion, and a third intermediate diameter portion. 2.A melter assembly as set forth in claim 1 wherein said directed flow isa coherent stream of molten source material.
 3. A melter assembly as setforth in claim 1 wherein the third portion of the nozzle is longer thanthe first and second portions of the nozzle.
 4. A melter assembly as setforth in claim 3 wherein the length of the third portion of the nozzlein millimeters is approximately equal to 50 plus 50 times the diameterof the third portion divided by the diameter of the second portion.
 5. Amelter assembly as set forth in claim 1 wherein said heater comprises afirst induction coil generally around an upper portion of the crucible,the first induction coil being adapted for connection to a power supplyfor supplying current to the first induction coil.
 6. A melter assemblyas set forth in claim 5 wherein said heater further comprises a secondinduction coil generally around a lower portion of the crucible, thesecond induction coil being adapted for connection to another powersupply to heat the source material in the lower portion of the crucible.7. A melter assembly as set forth in claim 6 further comprising asusceptor disposed between the first and second induction coils and thecrucible.
 8. A melter assembly as set forth in claim 7 wherein thesusceptor has an upper portion including a plurality ofcircumferentially spaced fingers to inhibit induction heating by thefirst induction coil, and a lower portion to promote induction heatingof the susceptor.
 9. A melter assembly as set forth in claim 8 whereinsaid upper portion has at least two sections separated by at least oneradial gap that inhibits induction heating by the first induction coil.10. A melter assembly as set forth in claim 8 wherein said susceptorcomprises a conical intermediate portion and a lower outlet portion. 11.A melter assembly as set forth in claim 10 wherein said conicalintermediate portion and said lower outlet portion are separated by agap.
 12. A melter assembly as set forth in claim 10 wherein saidintermediate portion and said upper portion are separated by a gap. 13.A melter assembly as set forth in claim 1 further comprising a splashguard adapted to be lowered from the melter assembly into the crystalforming apparatus to protect the crystal forming apparatus during theflow of molten source material from the melter assembly into the crystalforming apparatus.
 14. A melter assembly as set forth in claim 1 incombination with said crystal forming apparatus, said crystal formingapparatus comprising a crystal puller adapted for growingmonocrystalline ingots according to the Czochralski method.
 15. A melterassembly as set forth in claim 1 in combination with said crystalforming apparatus, said crystal forming apparatus comprising a crystalpuller adapted for growing polycrystalline silicon for use in themanufacturing of solar cells.
 16. A melter assembly and crystal formingapparatus as set forth in claim 15 wherein said crystal puller isadapted for growing hollow silicon bodies according to the Edge-DefinedFilm Growth method.
 17. A melter assembly and crystal forming apparatusas set forth in claim 15 wherein said crystal puller is adapted forgrowing ribbons of polycrystalline silicon.
 18. A melter assembly as setforth in claim 1 in combination with said crystal forming apparatus,said crystal forming apparatus comprising a crucible adapted for formingcast polycrystalline silicon bodies.
 19. A melter assembly as set forthin claim 1 further comprising a melt flow guide for guiding the flow ofmolten source material from the crucible to the crystal formingapparatus.
 20. A melter assembly as set forth in claim 19 wherein saidmelt flow guide comprises a tube received through the nozzle.
 21. Amelter assembly as set forth in claim 19 wherein said melt flow guidecomprises a tube that receives the nozzle.
 22. A melter assembly as setforth in claim 19 wherein said melt flow guide comprises a heatingelement for heating the molten silicon during flow between the nozzleand the crystal forming apparatus.
 23. A melter assembly for supplying acharge of molten source material to a crystal forming apparatus for usein forming crystalline bodies, the melter assembly comprising: a housingmovable from a position above a first crystal forming apparatus toanother position above a second crystal forming apparatus; a cruciblelocated in the housing; a heater disposed relative to the crucible formelting solid source material received in the crucible, the heatercomprising a first induction coil generally around an upper portion ofthe crucible, the first induction coil being adapted for connection to apower supply for supplying current to the first induction coil; asusceptor for supporting the crucible and promoting heating of the solidsource material in the crucible, the susceptor having an upper portionand a lower portion that are spaced apart to allow independent controlof the heater.
 24. A melter assembly set forth in claim 23 wherein saidsusceptor comprises an intermediate portion between the upper and lowerportions, the intermediate portion being separated from the upperportion and lower portion by a respective gap.
 25. A melter assembly setforth in claim 23 wherein said heater further comprises a secondinduction coil generally around the lower portion of the susceptor, thesecond induction coil being adapted for connection to another powersupply for supplying current to the second induction coil and inductionheating the lower portion of the susceptor.
 26. A melter assembly setforth in claim 23 wherein said upper portion has a plurality ofcircumferentially spaced fingers separated by gaps in the upper portionto inhibit induction heating of the upper portion of the susceptor. 27.A melter assembly as set forth in claim 26 wherein said upper portionhas a least two sections separated by at least one radial gap thatinhibits induction heating by the first induction coil.
 28. A melterassembly as set forth in claim 27 wherein said upper portion has foursections and four radial gaps.